Iridescent materials and devices

ABSTRACT

An iridescent material comprises a diffraction grating incorporating a plurality of sheets ( 1 ) of a first substantially transparent material in parallel alignment and spaced away apart by quantities of a second substantially transparent material ( 2 ), the second material having a refractive index significantly lower than the refractive index of the first material, the plates being tilted at an angle θ to an axis X and the grating having a period d(X) in the direction of axis X, the angle θ and period d being such that a second diffraction grating is provided along an axis Z, perpendicular to axis X, the second diffraction grating having a period d(Z) and consisting of ten periods or less and whereby, the diffraction conditions of the grating can be simultaneously fulfilled by electromagentic radiation of a range of wavelength thereby causing iridescence. Materials according to the invention have applications in inter alia document security, anti-counterfeiting measures, artwork, gift wrapping paper and cards and clothing design.

This invention relates to iridescent materials and in particular,multi-layer iridescent materials exhibiting angle-controllediridescence.

Iridescent materials are known for the attractive colour changingproperties they exhibit in different lights and at different grazingangles.

The present invention aims to provide iridescent materials of novelstructure which can be adapted for use in a variety of applicationsincluding document security, decorative packaging, advertising logos,textile fibres and the like.

In accordance with the present invention there is provided an iridescentmaterial comprising a diffraction grating incorporating a plurality ofsheets of a first substantially transparent material in parallelalignment and spaced apart by quantities of a second substantiallytransparent material, the second material having a refractive indexsignificantly lower than the refractive index of the first material, theplates being tilted at an angle θ to an axis X and the grating having aperiod d (X) in the direction of axis X, the angle θ and period d (X)being such that a second diffraction grating is provided along an axisZ, perpendicular to axis X, the second diffraction grating having aperiod d (Z) and consisting of twenty five periods or less and whereby,the diffraction conditions of the second diffraction grating can besimultaneously fulfilled by waveforms of a range of wavelengths therebycausing iridescence.

Preferably, the waveforms are electromagnetic and have a range ofwavelengths between about 10⁻¹ m and 10⁻⁸. More preferably 10⁻³ m and10⁻⁴ m and most preferably 10⁻⁶ m and 10 ⁻⁷ m.

Suitable options for the first material include, but are not limited to,transparent polymeric materials and glasses having refractive indices inthe region of 1.5. Other transparent materials having refractive indicesof a similar order will no doubt occur to the skilled addressee and arenot beyond the scope of the invention. It is to be understood that therefractive indices of the first and second materials are not essentialto the invention, provided that the ratio of the refractive index of thefirst material to that of the second material is greater than 1 andpreferably close to, or greater than, about 1.5.

Optionally, the second material is air, alternatively, the first andsecond materials are each provided in sheet form and are interleaved toform a multilayer structure.

These multi-layer structures may be obtained using a variety of methodsincluding, but not limited to, stereolithographic techniques,micro-machining or holographic techniques.

Preferably, the second diffraction grating consists of fifteen periodsor less, most preferably between 5 and 9 periods. A preferred embodimentconsists of 7 periods.

Preferably d is in the order of 10⁻¹ to 10⁻⁸ m in size, more preferably10⁻³ m and 10⁻⁸ m, and where visible light is to be caused toiridescent, is in the order of 10⁻⁶ m and 10⁻⁷ m, most preferablybetween about 400 nm and 770 nm.

In another aspect, the invention provides an iridescent devicecomprising a plurality of alternating substantially planar layers of twosubstantially transparent materials having different refractive indices,a surface at least part of which is inclined to the normal to the planesof the layers, the thickness of the device being such that a line drawnthrough the device perpendicular to said part of the surface interactsno more than 25 of said alternating layers, whereby said part of thesurface is capable of displaying an iridescent effect.

Devices in accordance with this aspect may cause iridescence of variouswaveforms, including electromagnetic waves. Optionally one of thetransparent materials is air. Either or both of the transparentmaterials may comprise polymeric materials and/or glasses. For certainapplications, where the thickness of the planar layers are to beprovided on a microscopic scale, some methods for making such devicesinclude, but are not limited to, stereolithographic techniques,micro-machining or holographic techniques.

In a third aspect, the invention provides a document security devicecomprising one or more of the novel iridescent materials applied to thewhole or part of a surface of the document. In this aspect the documentmay, for example, be a private or confidential letter or report,alternatively, the document may be a bankers card or otheridentification card, the iridescent materials providing a counterfeitproof identifier for the card. In another alternative, the document maybe a bank note.

Optionally, the document security device of the invention may comprise aplurality of iridescent materials arranged to form a pattern, eachiridescent material causing iridescence at a different grazing angleand/or for waveforms of different wavelengths. Preferably, thewavelengths of radiation caused to iridescent are electromagnetic and inthe range 10⁻³ m to 10⁻⁸ m.

In a fourth aspect, the invention provides a decorative materialcomprising a surface layer incorporating one or more of the noveliridescent materials. The decorative materials may, for example, includewrapping papers, card, fabrics for clothing manufacture and the like.Optionally, the decorative material may be provided in the form offibres which may then be woven into fabrics. In this aspect, thewaveforms caused to iridescent are preferably electromagnetic withwavelengths in the range 10⁻⁶ m to 10⁻⁷ m, i.e., visible light.

Optionally, the surface layer may comprise a plurality of the noveliridescent materials arranged to form a pattern, each iridescentmaterial causing iridescence at a different grazing angle and/or fordifferent wavelengths of visible light.

It has been found that iridescent materials according to the firstaspect of the invention produce complex diffractive behaviour givingrise to angle controlled iridescence. These materials can be designed toproduce surfaces which exhibit iridescence only at near grazing angles.By placing the iridescent materials on suitable substrates, articles canbe provided which absorb waveforms at angles which do not produceiridescence thereby resulting in high contrast iridescent properties.

In any of the above embodiments of the invention, where the waveform iselectromagnetic, the iridescent material may comprise a pigmentedsubstrate. In such embodiments, the material may exhibit the colour orpattern of the substrate except where conditions for iridescence are metThe conditions of iridescence may be applied to the material, say, tocheck for authenticity of the document carrying the material.

The invention will now be further described by way of example withreference to the following Figures in which:

FIG. 1 shows a section through the diffraction grating portion of aniridescent material according to the present invention.

FIG. 2 shows the relative scattering efficiencies of the first (x-axis)and second (z-axis) diffraction gratings of the diffraction gratingportion of an iridescent material according to the present invention

FIG. 3 shows scattering centres for the diffraction gratings of FIG. 2.

FIG. 4 a and 4 b show momentum vectors for photons incident on anddiffracted from the diffraction gratings of FIG. 2.

FIG. 1 shows a series of transparent plates 1 of high refractive indexregularly arranged in parallel in a tilted stack at an angle θ to anaxis X. The period of the grating d (X) along axis X is defined by thedistance between the plates 1. Perpendicular to axis X is a second axis,Z. The period of the grating d (Z) along axis Z is defined by thedistance between points where the Z axis passes through adjacent plates1. Thus it can be seen that the number of periods on the Z axis islimited both by the width of the plates 1 and the angle θ.

The gratings in both the X and Z axis are able to interact,simultaneously diffracting incident light in each axis. This behaviourcan be modelled and predicted (as illustrated in FIG. 4( a)) byconsidering a lattice of discreet points in momentum space. Each pointrepresents the termination point of a momentum vector that can beapplied to an incident photon when it has been diffracted from eitheraxis. Since photon momentum must be conserved, the incident andrefracted photon momentum vectors must join two points on the lattice.Hence, lines connecting these points define photon momentum vectorswhich are possible solutions to the diffraction equations for the twogratings. Thus it can be deduced that for a photon diffracted from bothgratings, there is only one possible photon momentum value which solvesboth equations. For a given angle of incidence, the photon momentumvalue is k_(ph)=2π/λ_(ph), where λ_(ph) is the photon wavelength.

As will be understood by the skilled addressee, diffraction gratings areresonant structures whose properties result from the interaction oftheir diffractive elements, the less diffractive elements there are, theless easily are the conditions for resonance defined. The inventors havefound that, where a grating consists of fewer than ten periods, thebehaviour of the grating begins to change.

In terms of the momentum space diagram (FIG. 4( a)), the effect oflimiting the number of diffracting elements (or periods d(Z)) in thegrating is a broadening of the discreet points into lines as shown inFIG. 4( b). These lines may be joined by a variety of routes, forexample, centre-to-centre (B-to-E) reproducing the vector for theinfinite grating case shown in FIG. 4( a), or by routes such as A-to-For C-to-D. Thus it can be seen that for the angle of incidence,different value photon momentum vectors (k_(ph)) can be defined. Thevisible result is that photons of several different wavelengths (as maycommonly be found in visible light) can strike the iridescent materialof the invention at the same angle and each will be diffracted at adifferent angle from the others. This results in a variably coloured huereturning from the material which is perceived by a viewer asiridescence.

The extent to which this iridescent effect is achieved is defined interms of the scattering intensity of the structure which is proportionalto Φ where:φ=[sin²(N.k _(ph) .d/2)]/[sin²(k _(ph) .d/2)]

FIG. 2 compares this quantity for a seven period (z axis) grating withthat for a 100 period (x axis) grating. As can be seen, the x axisgrating is a much more efficient scatterer (its efficiency being dividedby two orders of magnitude in the graph of FIG. 2) yet the z axisgrating scatters (diffracts) a much wider range of wavelengths of light.As can be predicted from the Figure, if the x axis grating were ofinfinite extent, then only a single wavelength of light would bediffracted.

Referring back to FIG. 4( b), the momentum width of the scattering peakin FIG. 2 can be estimated from the lengths of the vectors A-to-C andD-to-F since these represent the extent to which points in the momentumspace are blurred into lines by the unusual behaviour of a grating witha limited number of periods.

Thus it can be appreciated that a surface carrying an iridescentmaterial in accordance with the present invention will exhibit differentoptical effects when viewed from angles close to perpendicular to the Xaxis than when viewed at those close to perpendicular to the Z axis. Inthe former case, only very few wavelengths of light will be diffractedgiving a reflected image in a single colour, in the latter case a largernumber of wavelengths will be diffracted reflecting a multi-coloured,iridescent hue to the viewer. The effect can be used to disguise imagesso that they may be viewed clearly only from certain angles.

Possible applications of this technology include the coating of privateor confidential letters or documents so that they may only be viewed bythe reader positioned directly in front of the document. Similarly, dataon bankers or other identification cards may be disguised or madedifficult to counterfeit by use of these materials as surface coatings.Other anti-counterfeiting applications may include the application ofthese materials to bank notes so that certain images or information aremade iridescent only in certain lights.

Other applications may make use of the aesthetic qualities of theoptical effects produced by these materials to create eye-catching artwork or advertising materials. Similarly, attractive wrapping papers orgift cards may be produced. Where the invention is applied to fabrics,attractive clothing or soft furnishings may be produced. Other similarembodiments may occur to the skilled addressee without departing fromthe true scope of the invention.

1. A document security device comprising a document onto whichinformation can be provided and having applied to the whole or part of asurface of said document one or more iridescent materials, the one ormore iridescent materials each comprising a diffraction gratingincorporating a plurality of sheets of a first substantially transparentmaterial in parallel alignment and spaced apart by quantities of asecond substantially transparent material, the second material having arefractive index significantly lower than the refractive index of thefirst material, the sheets being tilted at an angle θ to an axis X andthe grating having a period d(X) in the direction of axis X, the angle θand period d (X) being such that a second diffraction grating isprovided along an axis Z, perpendicular to axis X, the seconddiffraction grating having a period d (Z) and consisting of twenty fiveperiods or less and at least five periods whereby, the diffractionconditions of the second diffraction grating can be simultaneouslyfulfilled by waveforms of a range of wavelengths thereby causingiridescence such that information which may be provided on the documentcan only be viewed clearly from a viewing point substantiallyperpendicular to the surface of the document.
 2. A document securitydevice as claimed in claim 1 wherein a plurality of iridescent materialsare applied to the surface of the document and are arranged to form apattern, each iridescent material causing iridescence at a differentgrazing angle and/or for different wavelengths of electromagneticradiation.
 3. A document security device as claimed in claim 1 whereinthe materials are configured for causing iridescence for electromagneticradiation having a wavelength in the range 10⁻³ m to 10⁻⁸ m.
 4. Adocument security device as claimed in claim 1 wherein the secondmaterial of the diffraction grating is air.
 5. A document securitydevice as claimed in claim 1 wherein the first and second materials ofthe diffraction grating are provided in sheet form and are interleavedto form a multilayer structure.
 6. A document security device as claimedin claim 1 where the number of periods is fifteen or less.
 7. A documentsecurity device as claimed in claim 6 wherein the second diffractiongrating consists of between 5 and 9 periods.
 8. A document securitydevice according to claim 1 wherein the document is a banker's card oran identification card.
 9. A document security device according to claim1 wherein the document is a bank note.
 10. A method of making a documentaccording to claim 1, said method comprising the step of forming aniridescent material by the steps of: (i) providing first and secondmaterials in sheet form; and (ii) interleaving the first and secondmaterials to form a multilayer structure.